1. Field of the Invention
The present invention relates to a pass/fail judgment device, a pass/fail judgment program, a pass/fail judgment method, and a multivariate statistics analyzer.
2. Description of the Prior Art
At plants for manufacturing various types of products, usually, pass/fail check is carried out before products are shipped. There are a variety of pass/fail check items. Dependence on humans' visual observation will make it difficult to check a large quantity of products at high speed and increase cost. Especially, if there are a great many check items, conducting visual pass/fail check is virtually impossible. To cope with this, a variety of pass/fail judgment devices which automatically carry out pass/fail check on various types of products have been provided.
Automation of pass/fail judgment involves a problem with respect to pass/fail judgment devices. In general, it is very difficult to clearly define what state of product should be considered as passed state or failed state based on objective criterion when a judgment device is constructed. When a device is constructed, a pass/fail judgment rule might be specified from a personal point of view. Even so, the rule is rarely applicable to every state and the pass/fail judgment device is rarely usable. Pass/fail judgment on test objects which can have a wide variety of product form, such as inspection of the state of soldering of mounted components, is especially difficult.
FIG. 22 is an explanatory drawing illustrating various types of the state of various components soldered onto a print circuit board (hereinafter just a board). The drawing shows the components viewed from the side of the board. In the uppermost column of the figure, a resistance element 1 favorably soldered onto a board is depicted on the left side and an element 1 with its contact lifted off on the right side. In the favorable state on the left side, the upper face of the solder 1a is recessed. In the lift-off state on the right side, the upper face of the solder 1b is projected. When the favorable state and the lift-off state are compared with each other, it is revealed that these pieces of solder are different in form at least the ends 1c and 1d of the solder. It is required to gather data sufficient to characterize this difference in form. Further, it is required to specify a pass/fail judgment rule which characterizes this difference in form.
In the second column of FIG. 22, a resistance element 1 favorably soldered onto a board is depicted on the left side and an element 1 with solder missing on the right side. When the favorable state and the solder missing state are compared with each other, it is revealed that they are obviously different in the form and quantity of solder in solder placement positions 1e. Further, in the third column of FIG. 22, a resistance element 1 and a resistance element 2 different in size from the resistance element 1 are depicted as soldered onto respective boards. Both the elements are favorably soldered. However, since the resistance elements 1 and 2 are different in size, they are also different in the quantity of solder and the inclination of the upper faces thereof. The upper face of the solder 2a on the resistance element 2 is more steeply inclined.
In the lowermost column of FIG. 22, mounted components 3 and 4 different in form are depicted as soldered onto respective boards. Again, both the components are favorably soldered. However, since the mounted components 3 and 4 are different in form, they are also different in the quantity of solder and the angle of the upper faces thereof. As mentioned above, the form or quantity of solder differs depending on whether the solder is good or bad and the form of the components. With respect to actual soldering, the form of the solder varies even with the same phenomenon, for example, the same lift-off. When a pass/fail judgment device is operated, it is required to gather data sufficiently to characterize the differences in the form of solder. Further, it is required to clarify differences in form based on the data and specify a pass/fail judgment rule for judging pass and fail with reliability.
More specifically, for the individual above-mentioned states, it is required to gather data at different points and specify pass/fail judgment rules for the different points. In case of soldering as illustrated in FIG. 22, it is thought that there are several hundred points (e.g. 200 points) in form which are so characteristic as to judge the acceptability of solder. It is required to extract from these points features which allows precise pass/fail judgment according to the kind of defect and the kind of components. However, it is impossible in practice to extract only appropriate ones from such a great many characteristic forms to specify pass/fail judgment rules. The reason therefor is that: usually, a great many components are mounted on a board, and several hundreds of features are present at the soldering points therefor. In this state, it is virtually impossible to artificially extract appropriate characteristic forms.
To cope with this, pass/fail judgment devices which perform statistical processing with personal points of view avoided as much as possible have been conventionally provided. For example, a pass/fail judgment device which uses discriminant analysis has been provided. The device extracts characteristic forms appropriate to pass/fail judgment from a great many characteristic forms, and makes judgment. One example of the applications of discriminant analysis is that: predetermined measured data acquired from a pass/fail judgment device is converted into a large number of pass/fail judgment parameters (parameters which can represent characteristic forms). Histograms of these parameters are generated for pass category and for fail category. A discriminant function which defines a new variable Z is computed, and pass/fail judgment is made with whether the variable Z is “0” taken as a threshold. The variable Z is a variable which is, when the frequency distributions of pass category and fail category are produced for the variable Z, determined so that both the categories can be separated as much as possible. The variable Z is a linear combination of the above pass/fail judgment parameters.
If threshold discrimination is made with the variable Z=0 in the above-mentioned conventional pass/fail judgment device, high-performance pass/fail judgment is not always implemented. To enhance the performance of pass/fail judgment, know-how must be accumulated through visual observation and actual operation. Further, very fine adjustment on how to select the above parameters and the like must be repeated. In terms of performance, pass/fail judgment is required to minimize a rate of flowout and a rate of overcontrol. Rate of flowout is a rate at which defective articles are judged as passed and let out. Rate of overcontrol is a rate at which acceptable articles are judged as failed and contained. In the above-mentioned conventional example, the enhancement of performance in this sense is very difficult.
The above-mentioned pass/fail judgment on solder will be taken as an example. In this case, various parameters corresponding to various characteristic forms, such as lift-off and solder missing, are selected by discriminant analysis. Then, pass/fail judgment is made depending on whether the variable Z is greater than 0. In this discriminant analysis, Z=0 is a midpoint between the mean value of pass category and the mean value of fail category. Therefore, in threshold discrimination by Z=0, a threshold is determined regardless of rate of flowout or rate of overcontrol, and it cannot be adjusted in advance so that a desired rate of flowout or rate of overcontrol will be obtained. To judge whether a desired rate of flowout or rate of overcontrol has been obtained, the following must be done: it must be verified whether any defective unit is included in solders which were subjected to pass/fail judgment and judged as passed. Further, it must be verified whether any non-defective unit is included in solders which were subjected to pass/fail judgment and judged as failed.
Further, if a desired rate of flowout or rate of overcontrol is not obtained, the following must be done: the method for selecting the above parameters must be changed to modify the discriminant function itself. Then, pass/fail judgment must be made again, and the above-mentioned verification must be repeated. That is, to enhance the performance of a conventional pass/fail judgment device, pass/fail judgment must be repeated by a huge number of times. Further, know-how must be accumulated by trial and error, and appropriate parameters must be selected. In conventional pass/fail judgment devices, the judging capability cannot be enhanced unless pass/fail judgment is actually made by a huge number of times.
The following are materials which may be related to the art of the present invention:
1. JP-A No. 254501/1996 (Date of Publication: Oct. 1, 1996)
The patent application discloses an art for using discriminant analysis in pass/fail judgment on the form of solder.
However, if the techniques disclosed in the patent application are used to obtain a threshold for discriminating between pass and fail, there is a high probability that defectives are let out.
Meanwhile, the present invention is predicated on the distribution of defective after discriminant analysis. Under the present invention, thresholds are set based on the breadth of the distribution and control can be exercised so as to reduce the flowout of defectives.
2. JP-A No. 229644/1997 (Date of Publication: Sep. 5, 1997)
The patent application discloses an art for using cluster analysis in pass/fail judgment on the form of solder.
The cluster analysis and the discriminant analysis may be in common with each other in that both are multivariate analysis.
However, they are completely different from each other in specific techniques and they can be considered to virtually have no commonality.